A radiotracer method which measures rates of oxidation of methane to cell material, extracellular products, and carbon dioxide has been applied to two lakes and indicates that methane oxidation occurred in a narrow band where methane and oxygen occurred together in the water column. Oxidation rates of 1.0 µM hr−1 were recorded in a eutrophic lake; rates in a meromictic lake reached 0.15 µM hr−1. Usually a third of the carbon from oxidized methane was found in cell material and extracellular products and the rest was converted to carbon dioxide. This ratio was observed to change at very low oxygen concentrations.